It is known that in household appliances of the aforesaid type, temperature control in the above-zero refrigerating unit is ensured only indirectly, through a fluid-dynamic connection between the two units determined by a passage conduit, along which motorised choking means constituted by a device known as a “damper” are arranged; such device is controlled by an electromechanical or electronic thermostat which appropriately moves the choking means, while an independent control system driven by the temperature present in the freezer unit operates the cooling circuit compressor, for example on the basis of the temperatures measured near the evaporator of the circuit arranged close to the sub-zero temperature refrigerating unit (freezer unit); the choking means consist in at least one mobile shutter member carried by a support fittable within the air passage conduit between the two refrigerating units, and in motor means of the shutter member. The same control system of the compressor also operates a ventilator arranged upstream of the evaporator, which ensures forced air ventilation in both refrigerating units (ventilated or “no-frost” cycle).
The solution described above is not entirely satisfactory. Indeed, an accurate temperature control, in particular in the above-zero unit, which is also the unit most frequently opened by the user, is not always optimal, which may also cause the early perishing of the foods conserved within. Furthermore, the solution described above can in practice only be used in ventilated or “no-frost” refrigerators-freezers, in which the presence of the ventilator ensures the forced passage of air between the two refrigerating units.
Finally, the need of maintaining adequately cool the above-zero refrigerating unit, which is more frequently opened and which consequently “warms up” more frequently, forces to adopt a temperature control cycle which entails a high waste of energy.